Add private BViewPort class

* It simplifies putting regular layout-aware views or layout items into
  a BScrollView.
* Not quite complete yet: Height-for-width support is missing, but that
  also requires fixing BScrollView in this respect. Scroll bar auto-hide
  support would be nice as well.

headers/private/interface/ViewPort.h

@@ -0,0 +1,53 @@
+/*
+ * Copyright 2013, Haiku, Inc. All Rights Reserved.
+ * Distributed under the terms of the MIT License.
+ *
+ * Authors:
+ *		Ingo Weinhold <[email protected]>
+ */
+#ifndef _VIEW_PORT_H
+#define _VIEW_PORT_H
+
+
+#include <View.h>
+
+
+namespace BPrivate {
+
+
+class BViewPort : public BView {
+public:
+								BViewPort(BView* child = NULL);
+								BViewPort(BLayoutItem* child);
+								BViewPort(const char* name,
+									BView* child = NULL);
+								BViewPort(const char* name,
+									BLayoutItem* child);
+	virtual						~BViewPort();
+
+			BView*				ChildView() const;
+			void				SetChildView(BView* child);
+
+			BLayoutItem*		ChildItem() const;
+			void				SetChildItem(BLayoutItem* child);
+
+private:
+			class ViewPortLayout;
+			friend class ViewPortLayout;
+
+private:
+			void				_Init();
+
+private:
+			ViewPortLayout*		fLayout;
+			BLayoutItem*		fChild;
+};
+
+
+}	// namespace BPrivate
+
+
+using ::BPrivate::BViewPort;
+
+
+#endif	// _VIEW_PORT_H